Heat capacities, phase transitions and structural properties of cation-exchanged H-mordenite zeolites

Abstract

Heat capacities of hydrogen large port mordenite (H-LPM), H-LPM+Cu and Na-LPM+Cu samples have been measured in the temperature range 25–500°C before and after treatment at 400°C. The changes in weight of the samples with temperature were also examined by thermogravimetry (TG). A large change in heat capacity of a transition at 133°C appeared for the H-LPM sample before treatment. The measured heat capacity was 3.75 J g −1 K −1 that exceeds H-LPM+Cu (3.5 J g −1 K −1) and Na-LPM+Cu (3.27 J g −1 K −1) samples with transitions at 129 and 146°C, respectively. The transition entropies of the samples before treatment showed almost no changes. However, after treatment, the transition entropies of H-LPM, H-LPM+Cu and Na-LPM+Cu samples were 31, 15 and 0.06 kJ g −1 K −1, respectively. These values indicated that Na + and Cu 2+ exchanged H-LPM influenced the entropy as a result of interaction of these cations with the zeolite framework. This interaction caused a diminishing of the self-diffusion of these ions. This was evidenced by the observed changes in lattice parameters of the H-LPM sample when Na + and Cu 2+ exchange took place, as measured by XRD and FT-IR spectroscopic analysis of the samples, in the range 100–1700 cm −1. This confirmed that the changes in the lattice modes were related to the increase in the electrostatic interactions between the cations and the zeolite framework. The transition heat capacities of the samples after treatment were found to be in the order: Na-LPM+Cu>H-LPM+Cu>H-LPM.